Cant do standing orders if no BTC was in there to begin with.
It's a handy feature of MtGox actually. Set up your sell order and if the price is right it executes when your BTC deposit arrives.
Similarly - even with no USD - you can place a sell order and a buy order such that if you manage to sell - you may then have funds for your buy order to execute.
That you didn't know about this feature suggests the possibility that you put the order there and didn't realize it was still in place.